1. Field
The present disclosure relates to display apparatuses and more particularly, to light guide panels, surface light source apparatuses including the light guide panels, and/or flat panel displays including the surface light source apparatuses.
2. Description of the Related Art
Liquid crystal displays used in personal computers (PCs), computer monitors, liquid crystal display (LCD) TVs, mobile communication terminals, or the like are light-receiving displays that do not emit light by themselves but display images by selectively transmitting light irradiated from the outside. Thus, a backlight is included on a rear surface of the liquid crystal displays as a surface light source apparatus.
In the liquid crystal displays, light emitted from the surface light source apparatus transmits through a liquid crystal layer arranged between a pair of polarization plates that have transmission axes at right angles to each other. An image is displayed as the light that transmits through the liquid crystal layer is electrically turned on or off.
An absorption-type polarization plate is used as the polarization plates. In the absorption-type polarization plate, an iodine-colored uniaxially oriented polyvinyl alcohol film is used as a polarizer. A protection film such as triacetyl cellulose film or the like, and a coating layer formed of an acrylic resin, or a phase difference film such as norbornene or polycarbonate is formed on one or both sides of the polarizer.
The absorption-type polarization plate transmits only light in a direction of a transmission axis of the polarization plate and absorbs the other components of light. Thus, in principle, light usage efficiency thereof (light transmittance) does not exceed 50%. Moreover, considering that reflectivity of an inner surface of the absorption-type polarization plate is 4%, the light usage efficiency of the absorption-type polarization plate is 46% at the greatest. Thus, to achieve low power consumption by liquid crystal displays, an efficient use of the backlight and an improvement in luminance are desirable.
As one of the methods for solving the above-described problem, a reflective polarization plate that uses optical reflection and interference is known. The reflective polarization plate reflects a desired polarization component light and transmits polarization of the opposite property to the desired polarization component light.
An axis of the reflective polarization plate is adjusted such that only polarization in a transmission axis direction is transmitted so that the light transmitted through the reflective polarization plate remains the same as linear polarization, and at the same time, the absorbed polarization is reflected for reuse in the absorption-type polarization plate. Thus, light usage efficiency of light emitted from the backlight may be improved.
An example of the reflective polarization plate is a dual brightness enhancement film (DBEF) including refractive index isotropic layers and refractive index anisotropic layers that are alternately stacked. However, a DBEF requires polymer films of several hundreds of stacked layers in total in order to provide polarization over a visible region. Thus, precise control is needed and this increases manufacturing costs.
To improve light usage efficiency and polarization separation power more cost-effectively, a technique of using a polarization sensitive scattering element (PSSE) is being researched. For example, Prior Art 1 (Japanese Patent Publication No. Hei 11-502036) discloses a method in which a polarization component in a direction perpendicular to a transmission axis is scattered to the backside by using a PSSE, and a polarization state of a corresponding backside scattering component is converted by using a ing s of stacked lay
In addition, Prior Art 2 (Japanese Patent Publication No. 2009-047802) discloses a reflective polarization plate in which a birefringent body formed of fibers having birefringence is used as a PSSE. In the reflective polarization plate, a layer in which a refractive index in a cross-sectional direction of the birefringent body (ordinary ray refractive index) corresponds to a refractive index of a supporting medium (polarization layer A) and a layer (polarization layer B) in which a refractive index in a length direction of the birefringent body (ordinary ray refractive index) corresponds to a refractive index of a supporting medium are alternately stacked such that arrangement directions of the birefringent bodies cross each other. Accordingly, polarization separation with respect to light that is obliquely incident or diffused light is improved.
In addition, Prior Art 3 (Japanese Patent Publication No. 2006-517720) discloses a method of improving polarization separation efficiency by scattering only one component of polarization and emitting the same to the outside by integrating an isotropic resin layer, in which birefringence fibers are buried as a PSSE, into a light guide panel.
To reduce power consumption of the surface light source apparatuses, LEDs having a long life span and power consumption reduction effect are frequently used as a backlight.